[LLVMdev] alias set collapse and LICM

[Sanjoy Das]

> Each alias set currently keeps track of its AccessType (Ref/Mod/ModRef). Maybe we should not collapse Ref access sets with Mod ones?

AFAICT, then the read only call will have to be in two alias sets -- a
Ref alias set (also containing %a) and a Mod alias set (also
containing %b and %c).  The collapse is driven by the fact that a
alias sets are disjoint -- to not collapse I think we'll have to break
that invariant.

-- Sanjoy

>
>  -Hal
>
>>
>> One potential fix is to do N^2 alias queries (every mem operation in
>> the loop with every other mem operation) before hoisting a load, but
>> I
>> suspect AliasSetTracker exists mainly to make LICM faster than N^2,
>> so
>> I'm ruling this solution out.
>>
>> The solution I like: have LICM create two AliasSetTracker
>> objects.  The first one tracks all memory operations, as the alias
>> set
>> tracker object does now.  The second one only tracks mods.  When
>> hoisting a load we check if any alias set in the mod tracker aliases
>> the load location -- if not, the load is okay to hoist to the
>> preheader from the data dependence point of view.  In the worst case,
>> this is 2x more (than now) work for building the alias sets and no
>> more
>> extra work when checking if we can hoist a load.
>>
>> Before I dive into implementing this, I have a few questions:
>>
>>  * is this even the right problem to solve?  Or should I look at
>>  other
>>    passes for this optimization?
>>
>>  * do you think this will be too expensive?  If so, does it make
>>  sense
>>    to do this only on -O3?`
>>
>> -- Sanjoy
>>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory